Pixel current driver for organic light emitting diode displays

ABSTRACT

A pixel current driver comprises a plurality of thin film transistors (TFTs) each having dual gates and for driving OLED layers. A top gate of the dual gates is formed between a source and a drain of each of the thin film transistors, to thereby minimize parasitic capacitance. The top gate is grounded or electrically tied to a bottom gate. The plurality of thin film transistors may be two thin film transistors formed in voltage-programmed manner or five thin film transistors formed in a current-programmed ΔV T -compensated manner. Other versions of the current-programmed circuit with different numbers of thin film transistors are also presented that compensate for δV T . The OLED layer are continuous and vertically stacked on the plurality of thin film transistors to provide an aperture ratio close to 100%.

This application is a continuation application of U.S. patentapplication Ser. No. 10/468,319 filed Jan. 23, 2004 now abandoned, whichis the U.S. National Phase of PCT/CA02/00173 having an InternationalFiling Date of Feb. 18, 2002, which claims the benefit of U.S.Provisional Patent Application Ser. No. 60/268,900 filed on Feb. 16,2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a an organic light emitting diodedisplay, and more particularly to an a pixel current driver for anorganic light emitting display (OLED), capable of minimizing parasiticcouplings between the OLED and the transistor layers.

2. Description of the Prior Art

OLED displays have gained significant interest recently in displayapplications in view of their faster response times, larger viewingangles, higher contrast, lighter weight, lower power, amenability toflexible substrates, as compared to liquid crystal displays (LCDs).Despite the OLED's demonstrated superiority over the LCD, there stillremain several challenging issues related to encapsulation and lifetime,yield, color efficiency, and drive electronics, all of which arereceiving considerable attention. Although passive matrix addressed OLEDdisplays are already in the marketplace, they do not support theresolution needed in the next generation displays, since highinformation content (HIC) formats are only possible with the activematrix addressing scheme. Active matrix addressing involves a layer ofbackplane electronics, based on thin-film transistors (TFTs) fabricatedusing amorphous silicon (a-Si:H), polycrystalline silicon (poly-Si), orpolymer technologies, to provide the bias voltage and drive currentneeded in each OLED pixel. Here, the voltage on each pixel is lower andthe current throughout the entire frame period is a low constant value,thus avoiding the excessive peak driving and leakage currents associatedwith passive matrix addressing. This in turn increases the lifetime ofthe OLED.

In active matrix OLED (AMOLED) displays, it is important to ensure thatthe aperture ratio or fill factor (defined as the ratio of lightemitting display area to the total pixel area) should be high enough toensure display quality. Conventional AMOLED displays are based on lightemission through an aperture on the glass substrate where the backplaneelectronics is integrated. Increasing the on-pixel density of TFTintegration for stable drive current reduces the size of the aperture.The same happens when pixel sizes are scaled down. The solution tohaving an aperture ratio that is invariant on scaling or on-pixelintegration density is to vertically stack the OLED layer on thebackplane electronics, along with a transparent top electrode (see FIG.2). In FIG. 2, reference numerals S and D denote a source and a drainrespectively. This implies a continuous back electrode over the OLEDpixel. However, this continuous back electrode can give rise toparasitic capacitance, whose effects become significant when theelectrode runs over the switching and other thin film transistors(TFTs). Here, the presence of the back electrode can induce a parasiticchannel in TFTs giving rise to high leakage current. The leakage currentis the current that flows between source and drain of the TFT when thegate of the TFT is in its OFF state.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide to apixel current driver for an organic light emitting display(OLED),capable of minimizing parasitic couplings between the OLED and thetransistor layers.

In order to achieve the above object, a pixel current driver for OLEDlayer for emitting light according to the present invention comprises aplurality of thin film transistors (TFTs) each having dual gates and fordriving the OLED layer. A top gate of the dual gates is formed between asource and a drain of each of the thin film transistors, to therebyminimize parasitic capacitance.

Each of the thin film transistor may be an a-Si:H based thin filmtransistor or a polysilicon-based thin film transistor.

The pixel current driver is a current mirror based pixel current driverfor automatically compensating for shifts in the Vth of each of the thinfilm transistor in a pixel and the pixel current driver is formonochrome displays or for full color displays.

The dual gates are fabricated in a normal inverted staggered TFTstructure. A width of each of the TFTs is formed larger than a length ofthe same to provide enough spacing between the source and drain for thetop gate. Preferably, the length is 30 μm and the width is 1600 μm. Thelength and width of the transistors may change depending on the maximumdrive current required by the circuit and the fabrication technologyused. The top gate is grounded or electrically tied to a bottom gate.The plurality of thin film transistors may be two thin film transistorsformed in voltage-programmed manner or five thin film transistors formedin a current-programmed ΔV_(T)-compensated manner, or four or The OLEDlayer is vertically stacked on the plurality of thin film transistors.

With the above structure of an a-Si:H current driver according to thepresent invention, the charge induced in the top channel of the TFT isminimized, and the leakage currents in the TFT is minimized so as toenhance circuit performance.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and features of the present invention will become moreapparent by describing in detail preferred embodiments thereof withreference to the attached drawings in which:

FIG. 1 shows variation of required pixel areas with mobility for 2-T and5-T pixel drivers;

FIG. 2 shows a pixel architecture for surface emissive a-Si:H AMOLEDdisplays;

FIG. 3 shows a cross section of a dual-gate TFT structure;

FIG. 4 shows forward and reverse transfer characteristics of dual-gateTFT for various top gate biases;

FIG. 5A and FIG. 5B show an equivalent circuit for a 2-T pixel driverand its associated input-output timing diagrams;

FIG. 6A and FIG. 6B show an equivalent circuit for a 5-T pixel driverand its associated input-output timing diagrams;

FIG. 7 shows transient performance of the 5-T driver for threeconsecutive write cycles;

FIG. 8 shows input-output transfer characteristics for the 2-T pixeldriver for different supply voltages;

FIG. 9 shows input-output transfer characteristics for the 5-T pixeldriver for different supply voltages;

FIG. 10 shows variation in OLED current as a function of the normalizedshift in threshold voltage;

FIG. 11 shows a 2-T polysilicon based pixel current driver havingp-channel drive TFTs;

FIG. 12 shows a 4-T pixel current driver for OLED displays;

FIG. 13 shows a 4-T pixel current driver with a lower discharge time;

FIG. 14 shows a 4-T pixel current driver without non-linear gain;

FIG. 15 shows a 4-T pixel current driver that is the building block forthe full color circuit;

FIG. 16 shows a full color(RGB) pixel current driver for OLED displays;and

FIG. 17 shows a schematic diagram of the top gate and the bottom gate ofa dual gate transistor where the top gate is electrically connected tothe bottom gate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although amorphous Si does not enjoy equivalent electronic propertiescompared to poly-Si, it adequately meets many of the drive requirementsfor small area displays such as those needed in pagers, cell phones, andother mobile devices. Poly-Si TFTs have one key advantage in that theyare able to provide better pixel drive capability because of theirhigher mobility, which can be of the order of μ_(FE)˜100 cm²/Vs. Thismakes poly-Si highly desirable for large area (e.g. laptop size) VGA andSVGA displays. The lower mobility associated with a-Si:H TFTs (μ_(FE)˜1cm²/Vs) is not a limiting factor since the drive transistor in the pixelcan be scaled up in area to provide the needed drive current. The OLEDdrive current density is typically 10 mA/cm² at 10V operation to providea brightness of 100 cd/m²—the required luminance for most displays. Forexample, with an a-Si:H TFT mobility of 0.5 cm²/Vs and channel length of25 μm, this drive current requirement translates into required pixelarea of 300 μm², which adequately meets the requirements of pixelresolution and speed for some 3 inch monochrome display applications.FIG. 1 illustrates simulation results for the variation of the requiredpixel size with device mobility calculated for two types of drivers,which will be elaborated later, the 2-T and the 5-T drivers, wherein μ₀denotes a reference mobility whose value is in the range 0.1 to 1cm²/Vs. For instance, the area of the pixel for the 2-T driver (see FIG.5A) comprises of the area of the switching transistors, area of thedrive transistor, and the area occupied by interconnects, bias lines,etc. In FIG. 1, the drive current and frame rate are kept constant at 10μA and 50 Hz, respectively, for a 230×230 array. It is clear that thereis no significant savings in area between the 2-T and 5-T drivers butthe savings are considerable with increasing mobility. This stems mainlyfrom the reduction in the area of the drive transistor where there is atrade-off between μ_(FE) and TFT aspect ratio, W/L (Wide/Length).

In terms of threshold voltage (V_(T)) uniformity and stability, bothpoly-Si and a-Si:H share the same concerns, although in comparison, thelatter provides for better spatial uniformity but not stability(ΔV_(T)). Thus the inter-pixel variation in the drive current can be aconcern in both cases, although clever circuit design techniques can beemployed to compensate for ΔV_(T) hence improving drive currentuniformity. In terms of long term reliability, it is not quite clearwith poly-Si technology, although there are already products based ona-Si:H technology for displays and imaging, although the reliabilityissues associated with OLEDs may yet be different. The fabricationprocesses associated with a-Si:H technology are standard and adaptedfrom mainstream integrated circuit (IC) technology, but with capitalequipment costs that are much lower. One of the main advantages of thea-Si:H technology is that it has become low cost and well-establishedtechnology, while poly-Si has yet to reach the stage ofmanufacturability. The technology also holds great promise forfuturistic applications since good as deposited a-Si:H, a-SiN_(x):H, andTFT arrays can be achieved at low temperatures (≦120° C.) thus making itamenable to plastic substrates, which is a critical requirement formechanically flexible displays.

To minimize the conduction induced in all TFTs in the pixel by the backelectrode, an alternate TFT structure based on a dual-gate structure isemployed. In a dual gate TFT (see FIG. 3), a top gate electrode is addedto the TFT structure to prevent the OLED electrodes from biasing thea-Si:H channel area (refer to FIG. 2). The voltage on the top gate canbe chosen such so as to minimize the charge induced in the (parasitic)top channel of the TFT. The objective underlying the choice of thevoltage on the top gate is to minimize parasitic capacitance in thedriver circuits and leakage currents in the TFTs so as to enhancecircuit performance. In what follows, the operation of the dual-gate TFTis described, which will be central to surface emissive (100% apertureratio) AMOLED displays based on a-Si:H backplane electronics.

FIG. 3 illustrates the structure of a dual-gate TFT fabricated for thispurpose, wherein reference numerals S and D denote a source and a drainrespectively. The fabrication steps are the same as of that of a normalinverted staggered TFT structure except that it requires a sixth maskfor patterning the top gate. The length of the TFT is around 30 μm toprovide enough spacing between the source and drain for the top gate,and the width is made very large (1600 μm) with four of these TFTs areinterconnected in parallel to create a sizeable leakage current formeasurement. A delay time is inserted in the measurement of the currentto ensure that the measurement has passed the transient period createdby defects in the a-Si:H active layer, which give rise to atime-dependent capacitance.

FIG. 4 shows results of static current measurements for four cases:first when the top gate is tied to −10V, second when the top gate isgrounded, third when the top gate is floating, and lastly when the topgate is shorted to the bottom gate. With a floating top gate, thecharacteristics are almost similar to that of a normal single gate TFT.The leakage current is relatively high particularly when the top gate isbiased with a negative voltage. The lowest values of leakage current areobtained when the top gate is pegged to either 0V or to the voltage ofthe bottom gate. In particular, with the latter the performance of theTFT in the (forward) sub-threshold regime of operation is significantlyimproved. This enhancement in sub-threshold performance can be explainedby the forced shift of the effective conduction path away from thebottom interface to the bulk a-Si:H region due to the positive bias onthe top gate. This in turn decreases the effect of the trap states atthe bottom interface on the sub-threshold slope of the TFT.

It should be noted that although the addition of another metal contactas the top gate reduces the leakage current of the TFT, it canpotentially degrade pixel circuit performance by possible parasiticcapacitances introduced by vertically stacking the OLED pixel. Thus thechoice of top gate connection becomes extremely critical. For example,if the top gates in the pixel circuit are connected to the bottom gatesof the associated TFTs, this gives rise to parasitic capacitanceslocated between the gates and the cathode, which can lead to undesirabledisplay operation (due to the charging up of the parasitic capacitance)when the multiplexer O/P drives the TFT switch. On the other hand, ifthe top gates are grounded, this results in the parasitic capacitancebeing grounded to yield reliable and stable circuit operation.

The OLED drive circuits considered here are the well-knownvoltage-programmed 2-T driver and the more sophisticatedcurrent-programmed ΔV_(T)-compensated 5-T version (see FIGS. 5A and 6A).The latter is a significant variation of the previous designs, leadingto reduced pixel area (<300 μm), reduced leakage, lower supply voltage(20V), higher linearity (˜30 db), and larger dynamic range (˜40 dB).Before dwelling on the operation of the 5-T driver, the operation of therelatively simple voltage-driven 2-T driver is described. FIG. 5B showsinput-output timing diagrams of the 2-T pixel driver. When the addressline is activated, the voltage on the data line starts chargingcapacitor C_(S) and the gate capacitance of the driver transistor T₂.Depending on the voltage on the data line, the capacitor charges up toturn the driver transistor T₂ on, which then starts conducting to drivethe OLED with the appropriate level of current. When the address line isturned off, T₁ is turned off but the voltage at the gate of T₂ remainssince the leakage current of T₁ is trivial in comparison. Hence, thecurrent through the OLED remains unchanged after the turn off process.The OLED current changes only the next time around when a differentvoltage is written into the pixel.

Unlike the previous driver, the data that is written into the 5-T pixelin this case is a current (see FIG. 6A). FIG. 6B shows input-outputtiming diagrams of a 5-T pixel driver. The address line voltage,V_(address) and I_(data) are activated or deactivated simultaneously.When V_(address) is activated, it forces T₁ and T₂ to turn on. T₁immediately starts conducting but T₂ does not since T₃ and T₄ are off.Therefore, the voltages at the drain and source of T₂ become equal. Thecurrent flow through T₁ starts charging the gate capacitor oftransistors T₃ and T₅, very much like the 2-T driver. The current ofthese transistors start increasing and consequently T₂ starts to conductcurrent. Therefore, T₁'s share of I_(data) reduces and T₂'s share ofI_(data) increases. This process continues until the gate capacitors ofT₃ and T₅ charge (via T₁) to a voltage that forces the current of T₃ tobe I_(data). At this time, the current of T₁ is zero and the entireI_(data) goes through T₂ and T₃. At the same time, T₅ drives a currentthrough the OLED, which is ideally equal to I_(data)*(W₅/W₃), whichsignifies a current gain. Now if I_(data) and V_(address) aredeactivated, T₂ will turn off, but due to the presence of capacitancesin T₃ and T₅, the current of these two devices cannot be changed easily,since the capacitances keep the bias voltages constant. This forces T₄to conduct the same current as that of T₃, to enable the driver T₅ todrive the same current into the OLED even when the write period is over.Writing a new value into the pixel then changes the current driven intothe OLED.

The result of transient simulation for the 5-T driver circuit is shownin FIG. 7. As can be seen, the circuit has a write time of <70 μs, whichis acceptable for most applications. The 5-T driver circuit does notincrease the required pixel size significantly (see FIG. 1) since thesizes of T2, T3, and T4 are scaled down. This also provides an internalgain (W₅/W₃=8), which reduces the required input current to <2 μA for 10μA OLED current. The transfer characteristics for the 2-T and 5-T drivercircuits are illustrated in FIGS. 8 and 9, respectively, generated usingreliable physically-based TFT models for both forward and reverseregimes. A much improved linearity (˜30 dB) in the transfercharacteristics (I_(data)/I_(OLED)) is observed for the 5-T drivercircuit due to the geometrically-defined internal pixel gain as comparedto similar designs. In addition, there are two components (OLED and T₅)in the high current path, which in turn decreases the required supplyvoltage and hence improves the dynamic range. According to FIG. 9, agood dynamic range (˜40 dB) is observed for supply voltage of 20V anddrive currents in the range I_(OLED)≦10 μA, which is realistic for highbrightness. FIG. 10 illustrates variation in the OLED current with theshift in threshold voltage for the 2-T and 5-T driver circuits. The 5-Tdriver circuit- compensates for the shift in threshold voltageparticularly when the shift is smaller than 10% of the supply voltage.This is because the 5-T driver circuit is current-programmed. Incontrast, the OLED current in the 2-T circuit changes significantly witha shift in threshold voltage. The 5-T driver circuit described hereoperates at much lower supply voltages, has a much larger drive current,and occupies less area.

The pixel architectures are compatible to surface (top) emissive AMOLEDdisplays that enables high on-pixel TFT integration density foruniformity in OLED drive current and high aperture ratio. A 5-T drivercircuit has been described that provides on-pixel gain, high linearity(˜30 dB), and high dynamic range (˜40 dB) at low supply voltages(15-20V) compared to the similar designs (27V). The results describedhere illustrate the feasibility of using a-Si:H for 3-inch mobilemonochrome display applications on both glass and plastic substrates.With the latter, although the mobility of the TFT is lower, the size ofthe drive transistor can be scaled up yet meeting the requirements onpixel area as depicted in FIG. 1.

Polysilicon has higher electron and hole mobilities than amorphoussilicon. The hole mobilities are large enough to allow the fabricationof p-channel TFTs.

The advantage of having p-channel TFTs is that bottom emissive OLEDs canbe used along with a p-channel drive TFT to make a very good currentsource. One such circuit is shown in FIG. 11. In FIG. 11, the source ofthe p-type drive TFT is connected to Vdd. Therefore, Vgs, gate-to-sourcevoltage, and hence the drive current of the p-type TFT is independent ofOLED characteristics. In other words, the driver shown in FIG. 11performs as a good current source. Hence, bottom emissive OLEDs aresuitable for use with p-channel drive TFTs, and top emissive OLEDs aresuitable for use with n-channel TFTs.

The trade-off with using polysilicon is that the process of makingpolysilicon TFTs requires much higher temperatures than that ofamorphous silicon. This high temperature processing requirement greatlyincreases the cost, and is not amenable to plastic substrates. Moreover,polysilicon technology is not as mature and widely available asamorphous silicon. In contrast, amorphous silicon is a well-establishedtechnology currently used in liquid crystal displays (LCDs). It is dueto these reasons that amorphous silicon combined with top emissive OLEDbased circuit designs is most promising for AMOLED displays.

Compared to polysilicon TFTs, amorphous silicon TFTs are n-type and thusare more suitable for top emission circuits as shown in FIG. 2. However,amorphous silicon TFTs have inherent stability problems due to thematerial structure. In amorphous silicon circuit design, the biggesthurdle is the increase in threshold voltage V_(th) after prolonged gatebias. This shift is particularly evident in the drive TFT of an OLEDdisplay pixel. This drive TFT is always in the ‘ON’ state, in whichthere is a positive voltage at its gate. As a result, its V_(th)increases and the drive current decreases based on the current-voltageequation below:Ids=(μC _(OX) W/2L)(V _(gs) −V _(th))²  (in Saturation region)

In the display, this would mean that the brightness of the OLED woulddecrease over time, which is unacceptable. Hence, the 2-T circuits shownearlier are not practical for OLED displays as they do not compensatefor any increase in V_(th).

The first current mirror based pixel driver circuit is presented, whichautomatically compensated for shifts in the V_(th) of the drive TFT in apixel. This circuit is the 5-T circuit shown in FIG. 6A.

Four more OLED pixel driver circuits are presented for monochromedisplays, and one circuit for full colour displays. All these circuitshave mechanisms that automatically compensate for V_(th) shift. Thefirst circuit shown in FIG. 12 is a modification of the 5-T circuit ofFIG. 6A. (Transistor T₄ has been removed from the 5-T circuit). Thiscircuit occupies a smaller area than the 5-T circuit, and provides ahigher dynamic range. The higher dynamic range allows for a largersignal swing at the input, which means that the OLED brightness can beadjusted over a larger range.

FIG. 12 shows a 4-T pixel driver circuit for OLED displays. The circuitshown in FIG. 13 is a 4-T pixel driver circuit based on a currentmirror. The advantage of this circuit is that the discharge time of thecapacitor Cs is substantially reduced. This is because the dischargepath has two TFTs (as compared to three TFTs in the circuit of FIG. 12).The charging time remains the same. The other advantage is that there isan additional gain provided by this circuit because T₃ and T₄ do nothave the same source voltages. However, this gain is non-linear and maynot be desirable in some cases.

In FIG. 14, another 4-T circuit is shown. This circuit does not have thenon-linear gain present in the previous circuit (FIG. 13) since thesource terminals of T₃ and T₄ are at the same voltage. It stillmaintains the lower capacitance discharge time, along with the otherfeatures of the circuit of FIG. 8.

FIG. 15 shows another version of the 4-T circuit. This circuit is doesnot have good current mirror properties. However, this circuit forms thebuilding block for the 3 colour RGB circuit shown in FIG. 16. It alsohas a low capacitance discharge time and high dynamic range.

The full colour circuit shown in FIG. 16 minimizes the area required byan RGB pixel on a display, while maintaining the desirable features likethreshold voltage shift compensation, in-pixel current gain, lowcapacitance discharge time, and high dynamic range.

It is important to note that the dual-gate TFTs are used in theabove-mentioned circuits to enable vertical integration of the OLEDlayers with minimum parasitic effects. But nevertheless the circuitcompensates for the Vth shift even if the simple single-gate TFTs. Inaddition, these circuits use n-type amorphous silicon TFTs. However, thecircuits are applicable to polysilicon technology using p-type or n-typeTFTs. These circuits when made in polysilicon can compensate for thenon-uniformity of the threshold voltage, which is a problem in thistechnology. The p-type circuits are conjugates of the above-mentionedcircuits and are suitable for the bottom emissive pixels.

1. A pixel current driver for an organic light emitting diode (OLED)having an OLED layer for emitting light, comprising: an address line; adata line; and a plurality of thin film transistors (TFTs) forming acurrent mirror, each having dual gates and driving for the OLED layer,the plurality of thin film transistors comprising: a switch thin filmtransistor, a first node of the switch transistor being connected to thedata line and a first gate of the dual gates of the switch transistorbeing connected to the address line; a feedback thin film transistor, afirst node of the feedback transistor being connected to the data lineand a first gate of the dual gates of the feedback transistor beingconnected to the address line; a reference thin film transistor, a drainof the reference transistor being connected to a second node of thefeedback transistor, a first gate of the dual gates of the referencetransistor being connected to a second node of the switch transistor anda source of the reference transistor being connected to a groundpotential; and a drive thin film transistor, a first gate of the dualgates of the drive transistor being connected to the gate of thereference transistor, a second gate of the dual gates of each thin filmtransistor being formed between a back electrode of the organic lightemitting diode and the first gate of the respective thin filmtransistor.
 2. The pixel current driver according to claim 1 wherein thethin film transistors are amorphous silicon.
 3. The pixel current driveraccording to claim 1 wherein the thin film transistors arepolycrystalline silicon.
 4. The pixel current driver according to claim3, wherein each of the transistors is a p-channel thin film transistor.5. The pixel current driver according to claim 1, wherein the dual gatesare fabricated in a normal inverted staggered TFT structure.
 6. Thepixel current driver according to claim 1, wherein the second gate ofthe thin film transistor is grounded.
 7. The pixel current driveraccording to claim 1, wherein the second gate of the thin filmtransistor is electrically tied to the first gate of the thin filmtransistor.